When a pre-defined set of data, or `data set`, is to be transferred from a host system's internal mass storage, such as a hard disk, to a secondary backup device, such as a tape drive, the procedure is known as a `backup job`. In most known backup application software packages there are two basic methods by which such a backup job may be initiated; manually or scheduled.
In the case of manual backup, an operator enacts the backup by executing backup application software on the host system, which operates immediately to backup selected, stored data files to, for example, tape.
When a backup job is scheduled, an operator executes the backup application software and configures software to run the backup job at a pre-defined time, such as `every night at midnight` or `every Friday at 1:00 am`, or, in general, when the data to be backed up is not being accessed by anyone. From the moment that the job is set up, the backup software sits substantially idle until the backup date and time arrives. At this point, the software establishes communications with the backup device, for example over an interface and connecting bus (e.g. SCSI), and then transfers the data.
At the point that the host system starts to actually run the backup job, the process of completing the job may be prone to failure for many different reasons. These might include:
a communications error such as a software driver becoming corrupted or deactivated; PA1 a connectivity problem such as the backup device has its data communications path broken (e.g. the SCSI cable has been unplugged); PA1 the backup device is powered off (e.g. somebody has accidentally pulled out the mains cable); PA1 a hardware failure in the backup device; PA1 the backup medium is not present (e.g. the tape has been taken out); or PA1 the backup medium cannot be written to (e.g. it is write-protected, worn out or full). PA1 scheduling a backup system to enact a pre-backup check to verify that a proposed backup job can be completed; PA1 scheduling a backup system to enact a backup job to occur at a time after the pre-backup check; PA1 generating a warning message in the event the pre-backup check determines that there is likelihood that the backup job will not be successful.
If one of these errors occurs, then the backup job will not be completed and the operator will typically not know about it until he observes an error message on the host system console, usually first thing the following morning. By the time the operator reads the message, no backup job has been run successfully and the system's data has been left unprotected. It is then too late to start another backup job as the system is now in use for everyday work tasks.